A set of eight-bit compression code words that are used at the beginning of a data run comprising a pixel count and one or more color values, the first four bits of the first word being used to specify the format of the bytes in the following data stream and whether the following pixels are to be printed as the overlay or as the background, and the last four bits being used to specify printing hints.
It is convenient for a printer to receive color image data in compressed form, and to expand that data just prior to printing. The compressed data typically contains text, computer generated graphics and scanned images, and each needs to be compressed and printed differently. For example, text is likely to be compressed using run length encoding and is constant color, so one byte is all that is required to describe the color of all pixels. On the other hand, a scanned-in image will continuously change color, so will require a color byte for each pixel, may be compressed using a lossy algorithm like JPEG, and will need to be halftoned at the printer.
To accommodate such a diverse set of inputs, a printer will need to get printing hints, along with the data, to handle each efficiently. One method would be to provide each printing hint with the boundary, coordinates or a bit map, to define the area in which the printing hint is to be used. Such a set of printing hints would be, for example, an eight-bit word divided into four two-bit sections, each section therefore being able to specify four choices. The first section could specify that the following data is text, contone, graphics, etc. The second set could specify one of four halftone screens. The third could identify the decompression algorithm required. Etc.
A number of compression algorithms are available. The particular ones assumed for this application are for data streams that are either scanned-in color that are DCT (discrete cosine transform), such as JPEG (Joint Photographic Experts Group) and LZ (Lempel Ziv) encoded, or computer generated graphics and text that are run length encoded (RLC).
It is also common to print an overlay over a background. The overlay is usually text and the background could be a form or a scanned-in picture. A bit map could be used to direct data from the desired data source to the printer for each pixel.